5,725 research outputs found

### Experimental Limits on Weak Annihilation Contributions to b â†’ ulv Decays

We present the first experimental limits on high-q^2 contributions to charmless semileptonic B decays of the form expected from the weak annihilation (WA) decay mechanism. Such contributions could bias determinations of |V_(ub)| from inclusive measurements of Bâ†’X_ulÎ½. Using a wide range of models based on available theoretical input we set a limit of Î“_(WA)/Î“_(bâ†’u) <7.4% (90% confidence level) on the WA fraction, and assess the impact on previous inclusive determinations of |V_(ub)|

### New Bound on gamma from B^+- -> pi K Decays

A bound on the angle gamma of the unitarity triangle is derived using
experimental information on the CP-averaged branching ratios for the rare
decays B^+- -> pi^+- K^0 and B^+- -> pi^0 K^+-. The theoretical description is
cleaner than the Fleischer-Mannel analysis of the decays B^+- -> pi^+- K^0 and
B^0 -> pi^-+ K^+- in that the two decay rates differ only in a single isospin
amplitude, which has a simple structure in the SU(3) limit. As a consequence,
electroweak penguin contributions and strong rescattering effects can be taken
into account in a model-independent way. The resulting bound excludes values of
cos(gamma) around 0.6 and is thus largely complementary to indirect constraints
derived from a global analysis of the unitarity triangle.Comment: minor corrections, version to appear in Physics Letters

### Prospects for improved $\Lambda_c$ branching fractions

The experimental uncertainty on the branching fraction \b(\Lambda_c \to p
K^- \pi^+) = (5.0 \pm 1.3)% has not decreased since 1998, despite a much
larger data sample. Uncertainty in this quantity dominates that in many other
quantities, including branching fractions of $\Lambda_c$ to other modes,
branching fractions of $b$-flavored baryons, and fragmentation fractions of
charmed and bottom quarks. Here we advocate a lattice QCD calculation of the
form factors in $\Lambda_c \to \Lambda \ell^+ \nu_\ell$ (the case $\ell = e^+$
is simpler as the mass of the lepton can be neglected). Such a calculation
would yield an absolute prediction for the rate for $\Lambda_c \to \Lambda
\ell^+ \nu_\ell$. When combined with the $\Lambda_c$ lifetime, it could provide
a calibration for an improved set of $\Lambda_c$ branching fractions as long as
the accuracy exceeds about 25%.Comment: 8 pages, 2 figures, version to be published in Phys.\ Rev.\

### Non-factorizable effects in top quark production

The production of top-antitop pairs at the Fermilab Tevatron shows a
forward-backward asymmetry in which the top quark tends to follow the proton
direction, while the antitop tends to follow the antiproton direction. The
effect grows with increasing effective mass $m_{t \bar t}$ of the top-antitop
pair, and with increasing rapidity difference between the top and antitop. The
observed effect is about three times as large as predicted by
next-to-leading-order QCD, but with the same sign. An estimate of
non-factorizable effects based on a QCD string picture finds they are
negligible, but that small distortions of the $m_t$ spectrum are possible.
Tests for such effects, both at and above the level of this estimate, are
suggested.Comment: 7 pages, 1 figure. Added text and references. Submitted to Phys. Rev.

### Properties of the Charmed P-wave Mesons

Two broad charmed mesons, the D_0^* and D_1', have recently been observed. We
examine the quark model predictions for the D_0^* and D_1' properties and
discuss experimental measurements that can shed light on them. We find that
these states are well described as the broad, j=1/2 non-strange charmed P-wave
mesons. Understanding the D_0^* and D_1' states can provide important insights
into the D_{sJ}^*(2317), D_{sJ}(2460) states whose unexpected properties have
led to renewed interest in hadron spectroscopy.Comment: 7 pages. Some additional discussion and reference

### Electroweak Constraints from Atomic Parity Violation and Neutrino Scattering

Precision electroweak physics can provide fertile ground for uncovering new
physics beyond the Standard Model (SM). One area in which new physics can
appear is in so-called "oblique corrections", i.e., next-to-leading order
expansions of bosonic propagators corresponding to vacuum polarization. One may
parametrize their effects in terms of quantities $S$ and $T$ that discriminate
between conservation and non-conservation of isospin. This provides a means of
comparing the relative contributions of precision electroweak experiments to
constraints on new physics. Given the prevalence of strongly $T$-sensitive
experiments, there is an acute need for further constraints on $S$, such as
provided by atomic parity-violating experiments on heavy atoms. We evaluate
constraints on $S$ arising from recently improved calculations in the Cs atom.
We show that the top quark mass $m_t$ provides stringent constraints on $S$
within the context of the Standard Model. We also consider the potential
contributions of next-generation neutrino scattering experiments to improved
$(S,T)$ constraints.Comment: 10 pages, 4 figures, final corrected version to be published in
Physical Review

### On the Mixing of the Scalar Mesons $f_0(1370)$, $f_0(1500)$ and $f_0(1710)$

Based on a $3\times3$ mass matrix describing the mixing of the scalar states
$f_0(1370)$, $f_0(1500)$ and $f_0(1710)$, the hadronic decays of the three
states are investigated. Taking into account the two possible assumptions
concerning the mass level order of the bare states
$|N>=|u\bar{u}+d\bar{d}>/\sqrt{2}$, $|S>=|s\bar{s}>$ and $|G>=|gg>$ in the
scalar sector, $M_G > M_S > M_N$ and $M_G > M_N > M_S$, we obtain the
glueball-quarkonia content of the three states by solving the unlinear
equations. Some predictions about the decays of the three states in two cases
are presented, which can provide a stringent consistency check of the two
assumptions.Comment: revtex 10 pages, 1 eps figur

### Charmed Baryons with $J = 3/2$

The width of a recently discovered excited charmed-strange baryon, a
candidate for a state $\Xi_c^*$ with spin 3/2, is calculated. In the absence of
configuration mixing between the ground-state (spin-1/2) charmed-strange baryon
$\Xi_c^{(a)}$ and the spin-1/2 state $\Xi_c^{(s)}$ lying about 95 MeV above it,
one finds $\tilde \Gamma(\Xi^*_c \to \Xi_c^{(a)} \pi) = (3/4) \tilde
\Gamma(\Xi^* \to \Xi \pi)$ and $\tilde \Gamma(\Xi^*_c \to \Xi_c^{(s)} \pi) =
(1/4) \tilde \Gamma(\Xi^* \to \Xi \pi)$, where the tilde denotes the partial
width with kinematic factors removed. Assuming a kinematic factor for P-wave
decay of $p_{\rm cm}^3$, one predicts $\Gamma(\Xi^*_c \to \Xi_c^{(a)} \pi) =
2.3$ MeV, while the $\Xi^*_c \to \Xi_c^{(s)} \pi$ channel is closed. Some
suggestions are given for detecting the $\Sigma_c^*$, the spin-3/2 charmed
nonstrange baryon, and the $\Omega_c^*$, the spin-3/2 charmed doubly-strange
baryon.Comment: 11 pages, latex, 2 uuencoded figures sent separatel

### A prototype system for detecting the radio-frequency pulse associated with cosmic ray air showers

The development of a system to detect the radio-frequency (RF) pulse
associated with extensive air showers of cosmic rays is described. This work
was performed at the CASA/MIA array in Utah, with the intention of designing
equipment that can be used in conjunction with the Auger Giant Array. A small
subset of data (less than 40 out of a total of 600 hours of running time),
taken under low-noise conditions, permitted upper limits to be placed on the
rate for pulses accompanying showers of energies around $10^{17}$ eV.Comment: 53 pages, LaTeX, 19 figures, published in Nuclear Instruments and
Methods. Revised version; some references update

- â€¦